Cation ordering in Co-Mg olivine solid-solution series

Five Co-Mg olivines (Co_0.23Mg_1.77SiO₄, Co_0.4 Mg_1.6SiO₄, Co_0.92Mg_1.08SiO_4,Co_1.45Mg_o.55SiO₄, and Co_1.76Mg_0.24SiO₄) and Co₂SiO₄ have been systematically synthesized by a floating-zone method and investigated by X-ray diffraction methods in order to clarify the intracrystalline distribution of Co²⁺ and Mg²⁺ ions between two octahedral sites [M(l) and M(2)]. Anisotropic least-squares refinements of the structures including the cation occupancies indicate that Co²⁺ ions are strongly concentrated on the smaller M(l) octahedral sites over the Co-Mg olivine solid-solution series. The octahedral distortion for the M(l) site shows a larger increase with increasing Co²⁺ content, whereas that for the M(2) site shows a small increase. The intracrystalline distribution coefficient, K_D = [Mg_M(2)Co_M(2)]/[Mg_M(1)Co_M(2)], attains a maximum of K_D ⋍ 5.4 at about 50 mol% Co₂SiO₄.